1. Field of the Invention
The invention relates to the selective use of light sources and subjects having markedly strong (or markedly weak) light emission and absorption characteristics in certain corresponding spectral bands. By matching and mismatching illumination and absorption in certain bands, a spectrally matched (or mismatched) subject is caused to assume a distinctly different appearance based upon the illumination source used. Particular illumination sources and pigments are disclosed herein wherein a strong difference in appearance is achieved.
In one embodiment, an illumination source has narrow spectral band peaks, exemplified by certain types of fluorescent lamps. In such a source, a combination of narrow wavelength bands (typically three primary color wavelengths) when added normally simulate illumination from a broadband source such as sunlight, having a given color temperature. According to an inventive aspect, an illumination source as described is applied to a wavelength absorptive pigment that is matched to at least one narrow band in the source, by virtue of a band at which the pigment is strongly absorptive. The preferably narrow absorptive band of the pigment is at least partly complementary to one of the color peaks emitted from the lamp.
An exemplary narrow band illumination source for use according to the invention may have discrete spectral peaks at particular wavelengths at visible blue, green and red wavelength bands. When these spectral peaks are added at appropriate relative amplitudes, the illumination is perceived by the eye as substantially white broadband light. A blue peak at 440 nm±15 nm, a green peak at 544 nm±15 nm and a red peak at 611 nm±15 nm are provided. Preferably, the bands are added at energy levels that cause the sum of the three sources to appear as a nominal color, for example the white of sunlight. However the technique can also produce a shift in appearance for light that is otherwise balanced, provided that there is a contribution from plural narrow spectral bands.
A particular pigment having a nominal color when illuminated with a true broadband source is specifically matched to the narrow band illumination source as described. Preferably the pigment has an absorptive peak (i.e., a reflective spectral gap) that is sufficiently strong and sufficiently matched to the wavelength band of one of the illumination source peaks that the overall color or hue, from the summed proportions of reflected colors from the pigment, shifts substantially and noticeably based on whether the particular narrow band keying peak wavelength is present in the illumination source.
2. Prior Art
It is known that materials exhibit different hues (colors) when illuminated with a light source that is complementary to characteristic colors in the reflective spectrum of the colored material, versus a light source that is not complementary.
In daylight illumination conditions, namely under light from the sun, the full visible spectrum is substantially represented. In sunlight, a nominal range of colors is visible because the light energy is spread over the entire range of visible wavelengths. Under such conditions, the appearance of an illuminated subject Is determined substantially only by the pigmentation of the subject, which determines the reflective spectrum of the subject. Thus, in sunlight, a red pigmented object appears red, a blue pigmented object appears blue, etc. Having evolved in sunlight, humans are adapted to distinguishing among illuminated objects based on their coloration as illuminated by a white or broadband source.
The solar spectrum is not wholly broadband. There are various spectral absorption lines introduced in the solar photosphere (known as Fraeunhofer lines). Also, the emission spectrum of he sun has a general peak at a color temperature around 5800° K. The characteristic illumination spectrum or color of the sun is more yellow than some blue or hotter stars but not as red as some cooler stars. At different times of day and in different atmospheric conditions, the spectrum of sunlight may differ due to considerations such as diffraction and atmospheric dust, for example causing the sunrise and sunset to appear more red than noon sunlight. Notwithstanding these variations, a daylight illumination spectrum is substantially broadband. There is a generally equal distribution of light energy over the visible spectrum. The reflective spectrum of illuminated subjects substantially determines the color appearance of the subjects, and not any aspect of the illumination,
There are some instances in which colored illumination is employed for effect. In day to day lighting applications, colored illumination might be undesirable because the colored lighting causes a subject to appear abnormal or unnatural. In other applications, colored light might be used deliberately because it is considered to make certain subjects more appealing than they might appear under flat spectrum broadband (“white light”) illumination. Typically, colored or tinted illumination involves adjusting the relative power level of a source toward generally redder “warm” tones or toward generally cooler and possibly harsher or more revealing bluer tones.
A light source might be tinted sufficiently that objects that should look “white” assume the tint of the light source to some extent. The ability of a human subjectively to detect subtle tints is limited and fades over time. After a time of exposure to a tinted light source, the light source seems white. The tint level and hue of lighting can have various effects. Fresh meats may look more appealing in slightly red light. Fresh vegetables may be more appealing in green or yellow light. Persons may have a skin tone that looks healthier with a bit of extra red.
In order to be effective for the foregoing purposes, differences in the color balance of light sources need to be subtle. The desired effects (healthy appearance or the like) might be defeated if a situation occurred wherein an article was successively illuminated by one light source and then another with a different tint. Illumination might be used to alter the appearance of a subject in a more radical way. A particular tint could be used to reveal a certain color and to wash out or mask certain other colors.
The emission spectra of light sources is a much studied matter. This is particularly the case for fluorescent lamps because there is an opportunity to adjust the tint of the light source by selecting among particular phosphor compositions and proportions of different compositions used to coat the inside of the fluorescent lamp (typically an elongated tube). Different phosphors have different emission spectra, but for physical or chemical reasons, the spectra generally have characteristic wavelengths where the light emission is relatively stronger and other wavelengths that are weaker.
Illumination is classified as to color temperature, which is a measure of the extent to which the spectrum tends to blue or to red. Solar radiation has a nominal color temperature of 5800° K., which can be considered the color of daylight, although daylight varies over the course of a day from a “whiter” color distribution (perhaps bluer is more accurate) to a redder one. According to JIS Standard Z 9112 (1990), there are standard ranges of color temperature for fluorescent and other lamps. Two scales used are:
JIS ClassificationTcp (K)IEC Publ. 81 equivalentDaylight5700-7100DaylightDay White4600-5400(no equivalent)White3900-4500Cool WhiteWarm White3200-3700WhiteIncandescent Color2600-3150Warm White
The color temperature represents a measure of the wavelength of the peak energy in a distribution of light energy versus wavelength. However the spectral light energy distribution of a light source typically is not a continuous spectrum. The energy distribution of fluorescent lamp has peaks and gaps due to the emission characteristics of the individual phosphors that line the fluorescent lamp tube.
Ordinary fluorescent lamps have calcium halophosphate phosphors lining the lamp tube. These phosphors have relatively broad and continuous spectra. Their emission extends over a range of wavelengths with a relatively constant level of power versus wavelength. The emission of such phosphors at wavelengths longer than 600 nm is limited, tending to make the illumination relatively blue or white, compared to daylight, which is somewhat more yellow or reddish by comparison. Combinations with additional phosphors have been proposed to supply additional red illumination. The emissions of several phosphors are summed in an effort to better synthesize the color of daylight. Lamps constructed using this concept are wide-band spectrum lamps, although narrower band phosphors may be included in the mix to adjust the contour of the spectrum.
An alternative type of fluorescent lamp uses narrow emission band phosphors with spectral peaks at respective primary colors, and much lower power levels at other wavelengths. According the “Phosphor Handbook,” CRC Press, pp. 367-373, the perception of the human eye is such that most colors can be effectively reproduced by combining light energy from narrow blue, green and red spectral bands. Particular suggested color bands are centered at wavelengths 450, 540 and 610 nm. This is the concept used in video display devices that control the brightness of red, blue and green dots at each pixel position of a display screen.
By selecting and optimizing particular phosphor compositions and combinations used In a light source, the peak emissions wavelengths can be selected as to their center wavelengths. The proportionate light energy applied at the three peaks can be varied by choice of phosphors and their proportions. In this way, the spectral balance of light intended to simulate white light or daylight is adjusted. However the spectrum of the light is not broadband and actually is comprised of a set of wavelength peaks of relative amplitudes and wavelengths selected by the phosphors used and the recipe of concentrations of phosphors used in lining the lamp.